The present invention relates to a miniaturized electret microphone which employs an electret as a diaphragm or a back electrode and has unidirectional characteristics.
In a unidirectional electret microphone, sound also reaches a diaphragm from behind it and the velocity component of such sound is made to have suitable relationships with respect to the velocity component of sound reaching the front of the diaphragm, thereby obtaining unidirectional characteristics. In the prior art, for instance, as disclosed in FIG. 2 of our U.S. Pat. No. 4,281,222 issued on July 28, 1981, an air-permeable damper cloth is held in contact with a back electrode on the opposite side from the diaphragm so that the sound from behind may reach the back of the diaphragm due to the air permeability of the damper cloth. In order to obtain excellent unidirectional characteristics, the velocity component of the sound from behind has to be of a suitable value suitably controlled by adjusting an acoustic impedance of the damper cloth. In the prior art, the acoustic impedance of the damper cloth is controlled by adjusting the pressure with which the damper cloth is held between the back electrode and another holding member. For instance, the diaphragm, the back electrode, the damper cloth and so forth are disposed one by one in a case and the rear end portion of the case is staked to the back of a rear closing plate member of the case to assemble a microphone. The velocity component of the sound from behind is controlled by adjusting the force of staking the rear end portion of the case. Accordingly, desired characteristics are difficult to obtain with this method and, sometimes, the microphone has to be disassembled for reconstruction. Therefore, the conventional microphone structure is poor in productivity.
Usually, felt, nonwoven fabric, air-permeable foamed resin and so forth have been employed as the damper cloth. In the case of using cloth such as felt or the like, it is frayed into a fringe, making in difficult to uniformly control the velocity component of the sound. The nonwoven fabric is non-uniform in weave, so that it must be used in several layers; this increases the manufacturing costs and offers an obstacle against miniaturization of the microphone.